We propose to evaluate a new method of assessing Sluggish Cognitive Tempo (SCT), and to characterize its latent structure in relation to DSM-IV symptoms of ADHD. SCT is a syndrome that is distinct from Hyperactivity-Impulsivity and Inattention. It is associated with inattenttive types of ADHD. The original features of SCT were daydreams and seems sluggish or drowsy. The construct has been broadened descriptively refer to slow information processing, inconsistent working memory, difficulty maintaining alertness and arousal, difficulty maintaining orientation, and tendency toward slow movement and passivity (low initiative). Research into SCT has been constrained by unresolved measurement issues. No empirically-based method of assessing SCT is available. Most studies have used only 2-3 SCT items, a number far fewer than recommended for reliable measurement of a latent construct (Devellis, 2003). Another limitation is that SCT items are non-specific and may be related to a range of transitory states (fatigue, sleep deprivation, etc.) and other psychopathological conditions. Left uncorrected, this aspect of SCT items would spuriously increase the observed prevalence of SCT and the apparent comorbidity between SCT and internalizing disorders. The proposed study seeks to remedy this gap in the literature by evaluating an interviewer- based diagnostic interview, the Kiddie Sluggish Cognitive Tempo Interview (K-SCT). The K- SCT contains an expanded item pool (16 items). It prompts the interviewer to classify endorsed items as being either primary, or as secondary to another condition or to a transient state. We will use psychometric criteria, factor analysis, and latent class analysis to evaluate items and to explore what the nosological effects may be of introducing SCT into the ADHD symptom pool. ADHD-I has been criticized for being heterogeneous and for being incorrectly nested within the externalizing disorders. Such problems are the consequences of relying solely on the number of hyperactivity-impulsivity symptoms as the criterion for differentiating types. SCT items may make it possible to define ADHD-I in terms of a set of characteristic symptoms, rather than as children who display the same symptoms of inattention as ADHD-C, but fall short of 6 symptoms of hyperactivity-impulsivity. If SCT indeed characterizes the prototypical case of attention deficit disorder without hyperactivity, then latent modeling of an expanded pool of ADHD symptoms (ADHD+SCT) may aggregate cases into groups with more within-group similarities and fewer between-group similarities than is seen using the current system. This would facilitate the identification of genetic and treatment response differences among types. PUBLIC HEALTH RELEVANCE: The proposed study will evaluate a novel diagnostic procedure for Sluggish Cognitive Tempo (SCT) that overcomes several current limitations. It will also examine how SCT fits in with the existing ADHD symptoms in DSM-IV. The results may provide a guide for updating the diagnostic criteria for ADHD in a way that would enhance efforts to identify genetic and treatment response differences among the types.